I have a Mac, an iPad and OpenCPN. I got OpenCPN (running on the Mac) on the iPad for free!! It works really well though you'll have to get to grips with taps, swipes and other gestures. I'll add some that I found below.

Note: Works on my iPhone too. I intend to get a second hand iPod cheaply as a backup - that should work too.

Note: this uses a free app (VNC Viewer by RealVNC) on your iPad that supposedly goes with a server app for your Mac. However, the server app will only work if you are connected to the internet, which we will not be. The edition that works in your LAN without internet is expensive and we don't need it. So go ahead and use the client (in your iPad) and Screen Sharing in your Mac. Works fine!

You'll need both your Mac and your iPad on the same network - preferably an on board router. If you don't have a router, you might try using you Mac as a router, that might work. Look it up on the internet - tips and instructions abound.

• Launch System Preferences either by clicking the System Preferences icon in the Dock, or selecting System Preferences from the Apple menu.
• Click the Sharing preference pane in the System Preferences window.​
• Place a check mark next to the Screen Sharing service.​
• Click the Computer Settings button.​
• In the Settings pane, place a check mark next to 'VNC viewers may control screen with password.'​
• Enter a password to be used when a remote user attempts to connect to your Mac.​ Note: this password grants access TO your Mac screen. We need it to prevent rogue neighbors from attempting to log into your Mac by denying them access to your Mac!
• Click the OK button.​
• Select which users will be allowed access to your Mac's screen. You can select 'All users' or 'Only these users.' In this case, 'users' refers to Mac users on your local network. Make your selection.​
• If you chose 'Only these users,' use the plus (+) button to add the appropriate users to the list.​
• When you're finished, you can close the Sharing preference pane.

AFAIK you don't need to make an account with RealVNC but I did not try that so if you need to, by all means register with RealVNC. However, I don't “sign in” as the app tries to tell me. I just ignore that and go ahead.

• Tap on 'Bonjour' and your Mac should appear.
• Tap on your Mac.
• Enter the VNC password for VNC viewers you entered in your Mac
• Now you will see your Mac log-in screen. Enter userid and password for your Mac. It makes no difference if you're already logged-in to your Mac!! Preferably you are and have OpenCPN on screen (makes life easier).

I usually start op OpenCPN on my Mac (easier). Things like routing etc are also easier on the Mac as the popup windows in OpenCPN seem to appear at random places (often behind the OpenCPN chart window).

• There's a mouse pointer on the screen. Move it around with one finger and tap anywhere to click. Your finger need not be on the pointer, which makes is easier to see what you're doing. This takes some getting used to as you'll have to keep an eye on the pointer in stead of tapping on something.
• Using two fingers to zoom in and out works
• Using one finger to move the chart works by double tapping (tap-taphold&move)
• Pinching zooms your iPad, NOT OpenCPN. It zooms in/out on you Mac screen. Use two fingers up and down to zoom the OpenCPN chart on your Mac.

Try it out having your Mac an iPad side by side so you can see what is going on.

Remember you are controlling your Mac from your iPad. So I do the hard stuff (anything that requires pop-up windows) on the Mac and use the iPad for following my route and seeing AIS targets et cetera at the helm.

I have no Windows PC so I cannot give you advice about what to use on that. But any decent VNC server software should work with VNC Viewer. My main discovery is the ease of use of the VNC Viewer app and the fact that it is free.